A Single-Pole, Single-Throw (SPST) mercury switch

Alternative mercury switch design

A mercury switch (also known as a mercury tilt switch) is a switch whose purpose is to allow or interrupt the flow of electric current in an electrical circuit in a manner that is dependent on the switch's physical position or alignment relative to the direction of the "pull" of earth's gravity. A switch is a mechanical device used to connect and disconnect an electric Circuit at will Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. An electrical network is an interconnection of Electrical elements such as Resistors Inductors Capacitors Transmission lines Voltage EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Gravitation is a natural Phenomenon by which objects with Mass attract one another

Mercury switches consist of one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum The envelope may also contain air, an inert gas, or a vacuum. "Inert gases" is also used in a narrower sense for Noble gases An inert gas is any Gas that is not reactive with elements Gravity is constantly pulling the drop of mercury to the lowest point in the envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts, thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles allowing, for example, Single-Pole, Double-Throw (SPDT) operation. A switch is a mechanical device used to connect and disconnect an electric Circuit at will

Uses

Roll sensing

Tilt switches may be used for a rollover or tip over warning for construction equipment and lift vehicles operating in rugged off-highway terrain. There are several non-mercury types but few are implemented due to sensitivity to shock and vibration - causing false tripping. However devices resistant to this do exist.

Thermostats

Mercury switches were commonly used in bimetal thermostats. Bi-metal refers to an object that is composed of two separate Metals joined together A thermostat is a device for regulating the Temperature of a System so that the system's temperature is maintained near a desired setpoint The weight of the movable mercury drop provided some hysteresis by moving the bimetal spring slightly beyond the point it would normally assume, thereby holding the thermostat off slightly longer before flipping to the on state and then holding the thermostat on slightly longer before flipping back to the off state. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts.

Vending

Mercury switches are still used in mechanical systems that are controlled electrically where the physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines that have 'tilt alarms'. A vending machine is a machine that provides various snacks beverages and other products to consumers When the machine is rocked or tilted in an attempt to gain a product, the mercury switch activates, sounding an alarm.

Bombs

The INLA used a mercury tilt switch and also microswitches to trigger the car bomb which killed Airey Neave during the Troubles. The Irish National Liberation Army ( INLA; Arm Saoirse Náisiúnta na hÉireann in Irish is an Irish Republican, Left-wing paramilitary organisation A micro switch is a generic term used to refer to an Electric switch that is able to be actuated by very little physical force A car bomb is an explosive device placed in a car or other Vehicle and then detonated. Airey Middleton Sheffield Neave, DSO, OBE, MC, (23 January 1916 &ndash 30 March 1979 was a British soldier barrister and politician [1][2]Other paramilitary groups also used the device, including the Red Hand Defenders who used a mercury switch to detonate the bomb that killed Rosemary Nelson[3]. A paramilitary is a force whose function and organization are similar to those of a professional military force but which are not regarded as having the same status The Red Hand Defenders (RHD is a Northern Irish paramilitary group formed in 1998 and composed largely of Protestant hardliners from loyalist groups observing a cease-fire Detonation is a process of Supersonic Combustion in which a Shock wave is propagated forward due to energy release in a reaction zone behind it Rosemary Nelson ( September 4 1958 – March 15 1999) was a prominent Roman Catholic Northern Irish Solicitor. Additionally, mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices e. A land mine is an Explosive device designed to be placed on or in the ground to explode when triggered by an operator or the Proximity of a vehicle person In an Explosive, Pyrotechnic device or military Munition, a fuse (or fuze) is the part of the device that initiates function An Anti-handling device is an attachment to or integral part of a Landmine, designed to prevent tampering g. a notable variant of the VS-50 mine. The VS-50 is an Italian circular plastic cased anti-personnel blast mine.

Toxicity

Since mercury has toxic properties, defective devices containing mercury switches must be treated as chemical waste. Toxicity is the degree to which a substance is able to damage an exposed organism Chemical waste is a waste that is made from harmful chemicals (mostly produced by large factories Because of this, mercury switches have been eliminated in most applications; for example, modern thermostats tend to use thermistors or other electronic devices to measure the temperature and a relay provides the switching action formerly performed by the mercury switch. A thermistor is a type of Resistor with resistance varying according to its Temperature. A relay is an electrical Switch that opens and closes under the control of another Electrical circuit.

See also

• Mercury arc valve, a rectifier device intended for high electrical voltages/currents
• Mercury battery, an electrochemical battery
• Mercury coulometer, an electro analytical chemistry device that determines the amount of matter transformed during a mercury reaction
• Mercury probe, an electrical probing device to sample for electrical characterization
• Mercury swivel commutator, a electrical circuit, current-reversing switch using the element mercury
• Mercury-wetted relay

External links

• How to Adapt a Mercury Switch for Robot Control